Silicon carbide (SiC) is a semiconductor having a dielectric breakdown field intensity about ten times as much as that of silicon (Si), and further having excellent physical values on thermal conductivity, electron mobility and band gap. Accordingly, silicon carbide has been expected as a semiconductor material capable of improving capability of semiconductor elements greatly as compared with conventional Si power semiconductor elements.
Recently, 4H-SiC and 6H-SiC single crystal substrates having a diameter up to 3 inches are on sale, and various kinds of semiconductor switching elements having capabilities remarkably higher than the limit of the Si capability have been reported one after another and the development of high capability SiC semiconductor elements has been progressed.
Semiconductor elements are roughly classified into unipolar semiconductor elements such that only electrons or holes act on electric conduction at the time of on-state forward bias operation, and bipolar semiconductor elements such that both of electrons and holes act on electric conduction at the time of on-state forward bias operation. The unipolar semiconductor elements include schottky barrier diode (SBO), junction field-effect transistor (J-FET), metal/oxide film/semiconductor electric field effect transistor (MOS-FET) and the like. The bipolar semiconductor elements include pn diode, bipolar junction transistor (BJT), thyristor, GTO thyristor, insulated gate bipolar transistor (IGBT) and the like.
When a power semiconductor element is produced using SiC single crystal, a single crystal film having a predetermined film thickness and a predetermined doping concentration is epitaxially grown on a SiC bulk single crystal substrate in the crystal polytype same as the substrate polytype in many cases, because the SiC single crystal has a very small diffusion coefficient and thereby is hardly possible to diffuse impurities deeply (Patent Document 1). Specifically, used is a SiC single crystal substrate, which is prepared by slicing a bulk single crystal obtained with a sublimation method or a chemical vapor deposition method (CVD) and has an epitaxial single crystal film grown with the CVD method on the surface of the substrate.
Examples of SiC single crystals may include various poly type ones (polymorphism type ones). 4H-SiC having high dielectric breakdown field intensity and mobility, and having relatively low anisotropy is used mainly. Examples of the crystal surface on which epitaxial growth is carried out may include (0001)Si surface, (000-1)C surface, (11-20) surface, (01-10) surface and (03-38) surface. When epitaxial growth is carried out on (0001)Si surface and (000-1)C surface, the crystal surface inclined at a several degree to the [11-20] direction or [01-10] direction is used in many cases.    Patent Document 1: Pamphlet of International Publication WO03/038876    Non-Patent Document 1: Journal of Applied Physics Vol. 95 No. 3 2004 pp. 1485 to 1488    Non-Patent Document 2: Journal of Applied Physics Vol. 92 No. 8 2004 pp. 4699 to 4704    Non-Patent Document 3: Journal of Crystal Growth Vol. 262 2004 pp. 130 to 138
As described above, power semiconductor elements prepared using SiC have excellent various properties, but have the following problems. In a SiC single crystal of a SiC bipolar semiconductor element, various crystal defects occur during the production process thereof. Specifically, in the first place, in the process of growing a SiC bulk single crystal by an improved Lely method or the CVD method, various crystal defects occur. In a SiC bipolar semiconductor element prepared using a wafer cut from a SiC bulk single crystal containing such various defects, the crystal defects present in the wafer cause lowering of the properties of the element.
In the second place, in the process of growing a SiC epitaxial film on the surface of a SiC bulk single crystal substrate by the CVD method, various crystal defects are generated in the SiC epitaxial film. One example of the crystal defects is basal plane dislocation.
FIG. 1 is a section showing an interfacial neighborhood between a SiC single crystal substrate and an epitaxial film grown on the surface of the substrate by a step flow growth technique. In FIG. 1, 5 is a crystal surface ((0001)Si surface), and θ is an off angle. As shown in the figure, many basal plane dislocations 3 which are one of crystal defects are present on the SiC single crystal substrate 1. For example, on a SiC single crystal substrate inclined at an off angle of 8° from the (0001) Si surface, the density of the basal plane dislocation on the substrate surface, which depends on the crystal quality, is typically from 102 to 104/cm2.
The basal plane dislocations 3 extended parallel to the (0001)Si surface emerge to the surface from the SiC single crystal substrate 1, and then about several % of the basal plane dislocations 3 propagate into an n type epitaxial film 2a and a p type epitaxial film (or a p type implanted layer) 2b as they are at the time of epitaxial growth, and the residual basal plane dislocations 3 are converted to threading edge dislocations 4 and then propagate into the n type epitaxial film 2a and the p type epitaxial film (or p type implanted layer) 2b. 
In a bipolar element such as pn diode or the like, the n type epitaxial film, and the interfacial neighborhood between the n type epitaxial film and the p type epitaxial film, or the interfacial neighborhood between the n type epitaxial film and the p type injection film become a region where electrons and holes are recombined at the time of on-state forward bias operation, but the basal plane dislocations 3 are converted into stacking faults by recombination energy of electrons and holes generated at the time of on-state forward bias operation (referred to the non-patent documents 1 to 3). The stacking faults occur as plane-like defects 31 having a triangle shape or the like, as shown in FIG. 4.
The basal plane dislocations have ⅓[10-20] Burgers vectors, but are present in a state of being divided into two Shockley partial dislocations of ⅓[10-10] and ⅓[01-10] (sometimes referred to Shockley type imperfect partial dislocations). Narrow regions, which are interspaces between these partial dislocations, form stacking faults. These stacking faults are called as Shockley type stacking faults. It is considered that one of the partial dislocations is moved by recombination energy of electrons and holes, and thereby the stacking fault area is enlarged.
Since the region of the stacking faults acts as a high resistant region at the time of on-state forward bias operation, the on-state forward voltage of the bipolar element is increased with the enlargement of the area of the stacking faults.
In the process for growing a SiC epitaxial film from the surface of a SiC single crystal substrate by the CVD method, various crystal defects are generated except for the basal plane dislocations. Specifically, crystal faults, for example, point defects, edge dislocations, screw dislocations and line defects with mixed component, loop-shape defects thereof occur in the SiC single crystal epitaxial film. Furthermore, it is considered that after forming the film by the CVD method, a strain will occur in the crystal at the time of decreasing the temperature, and crystal defects as described above will be generated at this time. It is further considered that particularly, many crystal defects as described above are present on the surface layer of the SiC epitaxial film.
As described above, since the inside of the SiC epitaxial film becomes a region where electrons and holes are recombined at the time of on-state forward bias operation, it is also considered that the above crystal defects will be converted into plane-like stacking faults by recombination energy of electrons and holes generated at the time of on-state forward bias operation. As described above, the stacking fault region acts as a high resistant region at the time of on-state forward bias operation, and thereby the voltage in the forward direction of the bipolar element is increased.
In the third place, after the SiC epitaxial film is formed on the surface of the SiC bulk single crystal substrate, the SiC bipolar semiconductor element is produced through various steps including mesa structure formation, ion implantation, oxide film formation, electrode formation, or the like. The above crystal defects also are generated in the step of processing the SiC single crystal substrate. For example, since the SiC bulk single crystal has a low constant of diffusing impurity atoms and it is difficult for the SiC bulk single crystal to apply impurity doping by thermal diffusion, nitrogen ion or aluminum ion is optionally introduced into the SiC epitaxial film by ion implantation. At the time of forming JTE in pn diode, ion implantation is also carried out for the SiC epitaxial film. It is considered that at the time of ion implantation, impurity ions derived into the crystal inside strike the SiC single crystal to destroy the crystal structure of the crystal, and thereby the SiC single crystal is damaged to cause occurrence of the above crystal defects.
As described above, various crystal defects are generated in the inside of the SiC single crystal in a step of forming the SiC single crystal substrate, in a step of forming the SiC epitaxial film and in a subsequent step of processing the SiC substrate. The crystal defects cause lowering of the properties of the SiC bipolar semiconductor element prepared, and particularly, the crystal defects present in the inside of the SiC epitaxial film by on-state forward bias operation become plane-like stacking faults. Furthermore, the area of the stacking faults is enlarged, to increase the voltage in the forward direction. The increase of the forward voltage lowers the reliability of SiC bipolar semiconductor element and induces increase of electric power loss in an electric power-controlling device equipped with the SiC bipolar semiconductor element. Therefore, there is a subject such that the forward voltage increased is recovered by shrinking the stacking faults enlarged by on-state forward bias operation.
The present invention is intended to solve the problems associated with the above prior arts, and it is an object of the present invention to recover the forward voltage increased in a silicon carbide bipolar semiconductor device by shrinking the stacking fault area enlarged by energizing.